The invention relates to an apparatus for determining and/or monitoring the viscosity of a medium in a container, having a unit which can oscillate, having a drive/reception unit and having a control/evaluation unit, in which case the unit which can oscillate is arranged at a defined measurement position within the container and/or in which case a unit which can oscillate is fitted such that it is immersed as far as a defined immersion depth in the medium, and in which case the drive/reception unit excites the unit which can oscillate to oscillate and/or in which case the drive/reception unit receives the oscillations from the unit which can oscillate.
Apparatuses are already known having at least one oscillating element, so-called vibration detectors, for detection and/or for monitoring the filling level of a medium in a container. The oscillating element is normally at least one oscillating rod, which is attached to a membrane. The membrane is stimulated to oscillate via an electromechanical transducer, for example a piezoelectric element. As a result of the oscillations of the membrane, the oscillating element which is attached to the membrane also oscillates. The xe2x80x9cLiquiphantxe2x80x9d, which is produced and marketed by the applicant, should be mentioned at this point as a very well known example of a vibration detector.
Vibration detectors designed as filling level measurement devices use the effect that the oscillation frequency and the oscillation amplitude are dependent on the respective extent to which the oscillating element is covered: while the oscillating element can carry out its (resonant) oscillations freely and without damping in air, it experiences a frequency and amplitude change, that is to say a change in tuning, as soon as it is partially or completely immersed in the medium. In consequence, on the basis of a predetermined frequency change (the frequency is normally measured for filling level identification), a clear conclusion can be drawn on whether the filling level of the medium in the container has reached a predetermined point. Filling level measurement devices are normally used primarily to provide protection against overfilling or to provide protection against pumps running dry.
The damping of the oscillation of the oscillating element is also influenced by the density of the medium. Thus, if the coverage level is constant, there is a functional relationship between the frequency change and the density of the medium, so that vibration detectors are highly suitable for determining not only filling levels, but also densities.
In practice, in order to monitor and identify the filling level and/or the density of the medium in the container, the oscillations of the membrane are recorded and are converted by means of at least one piezoelectric element into electrical response signals. The electrical response signals are then evaluated by evaluation electronics. For filling level determination, the evaluation electronics monitor the oscillation frequency and/or the oscillation amplitude of the oscillating element and signal the state (sensor covered) or (sensor uncovered) as soon as the measurement values fall below or rise above a predetermined reference value. An appropriate indication to the operator can be produced visually and/or audibly. Alternatively or additionally, a switching process is initiated; thus, for example, an inlet or outlet valve on the container is opened or closed.
The invention is based on the object of using a vibration detector for determining and/or monitoring the viscosity of a medium in a container.
The object is achieved in that the control/evaluation unit uses the frequency/phase curve of the unit which can oscillate to determine the viscosity of the medium. The present invention is based on the fact that the damping of a unit which can oscillate is dependent on the viscosity of the medium with which it is in contact. As is known, the term viscosity means the internal friction of a liquid, which is caused by attraction forces between the molecules. The viscosity is largely dependent on the parameters pressure and temperature.
The frequency/phase curves of a unit which can oscillate, which curves have been recorded in media with different viscosities, differ considerably from one anotherxe2x80x94as can clearly be seen from the graphs illustrated in FIG. 1: the lower the medium viscosity, the steeper is the drop in the frequency/phase curve. It has been found to be particularly advantageous to determine the viscosity of the medium using the frequency change which occurs for two different phase values. A relative measurement is thus carried out in preference to an absolute measurement. As will be explained in more detail in the following text, either two phase values are set for this purpose with the associated frequency change being determined, or the system passes through a predetermined frequency band to find out when at least two predetermined phase values are reached. The frequencies corresponding to the phase values are in turn used to determine the frequency change and, from this, the viscosity of the medium.
In FIG. 2, the viscosity is plotted against the frequency change for various phase shifts. A logarithmic scale was chosen. The curves can be described by the following mathematical formula: log xcex7=axc2x7logxcex94f+b, where a is virtually constant for all the curves, while the curves differ significantly in the constant b. In consequence, different phase shifts are reflected in a parallel shift in the frequency difference/viscosity curve along the frequency difference axis. The advantage of measuring the frequency change instead of the absolute frequency measurement is the increased measurement accuracy andxe2x80x94as will be described in detail in the following textxe2x80x94automatic elimination of disturbance variables, for example the density. The frequency change for a predetermined phase shift shows a clear dependency on the viscosity. In consequence, it is possible to determine the viscosity by determining the frequency difference for at least two predetermined phase values.
The influence of the density is visualized using the family of frequency/phase curves shown in FIG. 3 for a unit which can oscillate, in media with different densities:different densities lead to the frequency/phase curve being shifted parallel along the frequency axis. The higher the density, the lower is the oscillation frequency for the same phase value. The shape of the curves themselves is virtually identical in all cases. Since, according to the invention, relative values are measured rather than absolute values, the effect of changing density on the measurements is automatically eliminated.
According to a preferred development of the apparatus according to the invention, a piezo drive is used as the drive/reception unit. Piezo drives which can be used in conjunction with the present invention are known, for example, from EP 0 985 916 A1.
One advantageous development of the apparatus according to the invention provides for the drive unit to excite the unit which can oscillate to oscillate in a predetermined oscillation mode, in which case the oscillation mode is preferably the fundamental mode of the unit which can oscillate.
One preferred refinement of the apparatus according to the invention proposes that the control/evaluation unit have an associated memory unit in which data are stored which model the functional relationship between the frequency and the phase of the oscillations of the unit which can oscillate, for different damping conditions and for different viscosities. The data may be characteristics, formulae or measurement values.
The control/evaluation unit preferably sets at least two phase values which differ sufficiently from one another; after this, the control/evaluation unit determines the frequencies associated with the phase values and/or the corresponding frequency change in the oscillations of the unit which can oscillate, and determines the viscosity of the medium by comparing the frequency change which has already been found with the stored data.
According to one particularly advantageous refinement of the apparatus according to the invention, the at least two phase values are symmetrical with respect to the phase value xcfx86=90xc2x0.
One advantageous embodiment of the apparatus according to the invention provides for the control/evaluation unit to select the range in which the frequencies which are used to determine the viscosity are located such that the functional relationship between the phase values and the frequencies is essentially linear.
According to one alternative embodiment of the apparatus according to the invention, the control/evaluation unit sets at least two frequencies which differ from one another; the phases between the transmitted signal and the response signal associated with the frequencies of the oscillations of the unit which can oscillate are then determined; in a final step, the control/evaluation unit determines the viscosity of the medium by comparing the determined phase values and the stored phase values.
According to one preferred variant of the last-mentioned alternative of the apparatus according to the invention, the control/evaluation unit has an associated signal generator which actuates the drive unit such that the unit which can oscillate oscillates successively at different oscillation frequencies, in which case the oscillation frequencies are within a selected frequency band (xe2x86x92 frequency sweep).
Furthermore, one development of the apparatus according to the invention allows the unit which can oscillate to be designed as a universal detector; the control/evaluation unit thus drives the unit which can oscillate as a limit switch in a first operating mode, and as a viscosity sensor in a second operating mode. The respective operating mode is predetermined by a program contained in the control/evaluation unit.
An input/output unit is preferably provided, via which adjustments can be made on the apparatus, or via which information is provided relating to the measurement values which the apparatus supplies. At least one bus line is provided for interchanging data between the unit which can oscillate and a remotely arranged monitoring point. The data interchange itself can be carried out by means of any desired transmission standard, for example Profibus PA, Fieldbus Foundation.
The invention will be explained in more detail with reference to the following drawings, in which: